Vehicle speed, as displayed on or by a vehicle speedometer, may be determined by a controller based on a number of revolutions of a wheel, as indicated by a wheel speed sensor positioned at each wheel of the vehicle, over a predetermined period of time. A distance traveled over one wheel revolution varies based on wheel size (e.g., diameter), with the distance traveled over one wheel revolution increasing as the wheel size increases. The wheel size is known by the manufacturer of the vehicle during vehicle assembly, and so the vehicle speed calculation may be pre-calibrated based on the known wheel size.
As regulated by federal law, vehicle speed accuracy must be within 2.5% of the actual vehicle speed. Accuracy of the vehicle speed calculation depends on the accuracy of the wheel speed sensors. However, wheel speed sensors are susceptible to degradation and aging, which may reduce accuracy. Further, the addition of custom wheels to the vehicle, which may be a different size, a change in tire pressure, and other factors may reduce vehicle speed accuracy so that it is not within 2.5% of the actual vehicle speed even if the wheel speed sensors are accurate.
Other attempts to address vehicle speed inaccuracy include using global positioning system (GPS) data to determine the distance traveled by a vehicle over time, which gives vehicle speed, to recalibrate the vehicle speed calculation. One example approach is shown by Wurth et al. in U.S. Pat. No. 7,460,950 B2. Therein, GPS data are used to generate a reference vehicle speed, which is compared to an estimated vehicle speed (as calculated from wheel speed sensor data) and used to generate a corrected vehicle speed.
However, the inventors herein have recognized potential issues with such systems. As one example, GPS data are too inaccurate to reliably give vehicle speed within the 2.5% accuracy margin. For example, GPS accuracy may be affected by factors including atmospheric effects, sky blockage, and GPS receiver quality. GPS data are also less reliable at lower speeds due to errors in determining a change in position.
In one example, the issues described above may be addressed by a method for adjusting a determination of speed of a motor vehicle responsive to character recognition of an image from an on-board camera of the vehicle, the image including a speed measurement source external to the vehicle. For example, the determination of vehicle speed may be based on output from wheel speed sensors of the vehicle, and the speed measurement source may be a radar speed sign. In this way, an accurate vehicle speed may be generated.
As one example, a vehicle equipped with an on-board camera and radar system may recognize traffic sign boards, such as a radar speed sign, through an image recognition module. A value displayed on the radar speed sign may be determined from an image acquired by the on-board camera and used as an independent measurement of the vehicle speed (e.g., as a reference speed). Further, the value displayed on the radar speed sign may be used to determine a compensation to apply to the vehicle speed calculated using wheel speed sensors of the vehicle, thereby increasing the accuracy of the calculated vehicle speed. Increasing the vehicle speed accuracy may increase vehicle performance and prevent a driver of the vehicle from receiving traffic citations due to speed limit violations. Further, the method described herein may also be utilized by autonomous vehicles to check and correct vehicle speed.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.